1. Field Of The Invention
The present invention relates to semiconductor technology. More specifically, the present invention relates to one-time user-programmable anti-fuses which present an open circuit prior to programming, and a low-resistance connection after programming. The present invention comprises improved programming methods for such anti-fuse structures.
2. The Prior Art
One-time user-programmable anti-fuses which present an open circuit prior to programming, and a low-resistance connection after programming are known in the prior art. Such anti-fuses include two conducting electrodes which are separated from one another by a single layer or multilayer dielectric material and are programmed by placing across the electrodes a programming voltage of sufficient magnitude to rupture the dielectric layer and create a conductive filament therethrough.
It is known that anti-fuse resistance can be reduced by heavy arsenic doping of one or both anti-fuse electrodes. An example of such an anti-fuse is disclosed and claimed in U.S. Pat. No. 4,823,181 to Mohsen et al.
It is also known to improve anti-fuse structures of the type disclosed and claimed in the above-identified patent by providing that the arsenic doping of at least one electrode be performed such that a high concentration of arsenic atoms exists at the interface of the heavily doped electrode and the dielectric layer, a condition sometimes referred to as arsenic pile-up. An example of such an anti-fuse is disclosed and claimed in U.S. Pat. No. 4,899,205 to Hamdy et al.
The programming of both of these anti-fuses and other arsenic doped anti-fuse structures according to prior art techniques includes the step of applying a DC voltage across the two electrodes, the voltage being of sufficient magnitude to rupture the dielectric in between the electrodes to create a low resistance filament connecting the two electrodes together. A programming procedure for such anti-fuse structures is disclosed in co-pending application Ser. No. 381,630, filed Jul. 18, 1989. That application discloses that, after the initial application of one or more pulses of programming voltage which serve to rupture the dielectric layer between the two electrodes, pulses of programming voltage are repeatedly applied while the current through the anti-fuse is monitored in order to determine the resistance of the programmed anti-fuse. This procedure after initial dielectric rupture is sometimes referred to as "soaking" the anti-fuse, and it is performed in an attempt to lower the final anti-fuse resistance. When the resistance value has reached a desired level, or when the change in resistance between successive pulses drops below a predetermined level, the programming process is terminated.
Furthermore, in that co-pending application, the "soaking" voltage pulse has the same polarity as the programming pulse. No attempt was made to optimize the polarity of the soaking pulse.
While it is apparent to those of ordinary skill in the art that it is preferable to choose the programming voltage polarity to reduce the programming time, it is not apparent that the "soaking" voltage pulse polarity may also be chosen separately from the polarity of the programming pulse in order to lower final antifuse resistance.
It will be apparent to those of ordinary skill in the art that the higher the soaking current or the programming current, the lower the anti-fuse resistance will be.
For a given anti-fuse in the hands of the programmer, the arsenic concentration in the electrodes is fixed, and when the programming and soaking current level becomes relatively constant, it has heretofore been assumed that the programmed anti-fuse resistance has been minimized.